The present invention relates to an antenna device. More particularly, the present invention relates to a compound antenna device on which a plurality of antennas is mounted.
Various types of antennas are mounted on a vehicle. Examples of the antennas are: an antenna used for GPS (Global Positioning System); an antenna used for ETC (Electronic Toll Collection System); an antenna used for VICS (Vehicle Information Communication System); and an antenna for telephone use.
GPS (Global Positioning System) is a satellite positioning system using artificial satellites. In GPS, electric waves (GPS signals), which are sent from four (4) artificial satellites in twenty-four (24) artificial satellites geostationary, i.e., rotating with the earth, are received. According to the received electric waves, a positional relation between a moving body and the artificial satellites and an error of time are measured. According to the principle of trigonometric survey, a position and altitude of the moving body on a map can be highly accurate.
GPS is recently used for a car navigation system which detects a position of an automobile running. The car navigation system includes: an antenna used for GPS to receive GPS signals; a processor for processing GPS signals received by the antenna for GPS and for detecting a present position of the vehicle; and a display for displaying a position, which has been detected by this processor, on a map.
On the other hand, ETC (Electric Toll Collection) is a system developed as a measure for relieving traffic jam at a toll-booth in which a toll of a toll road such as a highway is collected. That is, ETC is a system for automatically paying the toll by utilizing wireless correspondence at the toll booth of the highway. ETC is operated as follows. Between a road side antenna provided in a gate arranged in the toll booth and an antenna for ETC of a vehicle, which passes through the toll booth, on which a corresponding device for ETC is mounted, bilateral communication is performed. In this way, the toll booth obtains a piece of necessary vehicle information of the vehicle passing through the toll gate. Therefore, the toll of the highway can be paid without stopping the vehicle.
The antenna for GPS and the antenna for ETC are jointly provided in a related-art compound antenna.
In a first related-art compound antenna device, a plurality of antenna elements are respectively mounted on corresponding boards and low noise amplifying circuit parts (LNA) are attached onto a lower side of each board. This low noise amplifying circuit parts (LNA) are covered with a shielding case.
For example, as shown in FIG. 4, a compound antenna 100 (refer to Japanese Patent Publication No. 2003-309411) is a small antenna unit mounted on a vehicle which is composed as follows. On an attaching plate, a patch antenna 103 for receiving circularly polarized waves sent from GPS satellite and a rod-shaped antenna 104 for sending and receiving linearly polarized waves used for mobile phone are arranged together and both the antennas are accommodated in a common radome 112. Electricity is fed to the patch antenna 103 through a cable 105 and a connector 113. The patch antenna 103 is composed as follows. A feeding patch 107, which is an emission element of a micro-strip structure, is provided on an upper face of a dielectric board 108. This dielectric board 108 is mounted on a circuit board 109 Low noise amplifying circuit parts (LNA) 116 such as an amplifier and a band-pass filter covered with a shield case 115 are mounted on a lower side of the circuit board 109.
In Japanese Patent Publication No. 2003-309411, the compound antenna is provided with a patch antenna and a rod-shaped antenna. It is sufficient to estimate the first related-art compound antenna device described above to have a structure in which a plurality of patch antennas are provided and each patch antenna is mounted on a board.
However, in the first related-art compound antenna device, each shielding case is respectively attached to an antenna element. Therefore, problems may be encountered when the size of the device is reduced and the number of parts is decreased.
In order to solve the above problems, in the second related-art compound antenna device, the low noise amplifying circuit parts (LNA) attached onto the lower side of each board are covered with one shielding case. Accordingly, the number of parts can be decreased and further the size of the device can be reduced.
However, according to the second related-art compound antenna device described above, the following problems may be encountered. Onto the lower side of the board on which each antenna element is respectively mounted, low noise amplifying circuit parts (LNA) are attached and the shielding cover is attached so that it can cover the entire LNA. Therefore, no shielding member is provided between one antenna (for example, an antenna for GSP) and the other antenna (for example, an antenna for SDARS (satellite radio)). Accordingly, earth interference with each antenna and LNA is caused.